The present invention relates to modular components of a prosthetic joint, and more particularly to modular components of a prosthetic knee joint.
The knee joint basically consists of the bone interface of the distal end of the femur and the proximal end of the tibia. Appearing to cover or at least partially protect this interface is the patella, which is a sesamoid bone within the tendon of the long muscle (quadriceps) on the front of the thigh. This tendon inserts into the tibial tuberosity and the posterior surface of the patella is smooth and glides over the femur.
The femur is configured with two knob like processes (the medial condyle and the lateral condyle), which are substantially, smooth and articulate with the medial plateau and the lateral plateau of the tibia, respectively. The plateaus of the tibia are substantially smooth and slightly cupped thereby providing a slight receptacle for receipt of the femoral condyles.
When the knee joint is injured whether as a result of an accident or illness, a prosthetic replacement of the damaged joint may be necessary to relieve pain and to restore normal use to the joint. Typically the entire knee joint is replaced by means of a surgical procedure, which involves removal of the ends of the corresponding damaged bones and replacement of these ends with prosthetic implants. This replacement of a native joint with a prosthetic joint is referred to as a primary total-knee arthroplasty.
Prosthetic knee implants include femoral implants that are designed to be affixed to the distal end of the resected femur. A known modular femoral implant is illustrated in FIGS. 1-3. As there shown, the modular femoral implant 10 includes a femoral component 12, an elongate stem member 14, bolt 16 (shown in FIG. 3), and a femoral stem collar 18.
The modular femoral component 12 includes two spaced condylar portions 20, 22 with articulating surfaces 24, 26 to engage an articulating surface of a tibial implant (not shown). On the proximal side of the femoral component 12, the two condylar portions 20, 22 are connected by an intercondylar box or pad 28. The intercondylar box or pad 28 has a pair of substantially vertical side walls 30, 32 that are connected by a top or superior seating or mounting platform 34. The pad mounting platform 34 is generally planar, and has an opening 36 (see FIG. 3) that extends through the pad to define an open channel. The bolt 16 extends through the channel from the distal side of the femoral component and through the opening 36 to be connected to a female threaded opening in the femoral stem 14.
The femoral stem collar 18 has a threaded male portion 37 (FIG. 3) for connection to a distal female threaded end of the stem member 14. Thus, the stem member 14, stem collar 18 and femoral component 12 can be assembled to secure the stem member 14 to the femoral component 12. With this design, a variety of styles and sizes of stem members and femoral components can be assembled to best suit the patient's anatomy and joint conditions. For example, an implant kit could include a set of different sizes of stem members with outer surfaces adapted for cemented implantation as well as with fluted outer surfaces.
As shown in FIG. 2, when assembled, the stem member 14 is typically angled in a medial direction. The angle is labeled in FIG. 2 as α. The angle α is between the axis 40 of the stem member 14 and a line 41 perpendicular to the plane 42 of the seating or mounting platform 34 of the intercondylar pad or box 28. The angle α corresponds with the valgus angle when the implant assembly is implanted; the valgus angle is defined as the angle between the center line of the femur and the vertical axis connecting the distal femur and the center of the femoral head; the center line of the femur will correspond with the axis 40 of the stem member 14, and the vertical axis connecting the distal femur and the center of the femoral head will correspond with the line 41.
In the illustrated prior art modular assembly, the angle α is set by the structure of the femoral stem collar 18. The femoral stem collar has a superior side or surface 33 lying in a plane and an inferior side or surface 35 lying in a plane that is not parallel to the plane of the superior side or surface 33. The inferior surface of the collar is angled, defining an obtuse angle (90°+α) with the axis 40 of the stem. In typical implant sets, a plurality of femoral stem collars 18 are provided, shaped so that the angle α can be set to be any one of a number of angles. In one implant set available from DePuy Orthopaedics, Inc. of Warsaw, Ind., the femoral stem collars are shaped to define angles α of either 5° or 7°, but angles α may be in a typical range of 5-9°.
Although not shown in FIGS. 1-3, the illustrated prior art stem collar 18 has a central bore to receive part of the bolt 16. The central bore has a central longitudinal axis defining an obtuse angle with at least one of the plane of the superior side 33 and inferior side 35 of the stem collar 18.
To ensure that the angle α remains in the illustrated orientation, the intercondylar box or pad 28 typically has a pair of anti-rotation tabs 46, 48 (see FIGS. 1 and 3) that mate with opposing flats 50, 52 on the femoral stem collars 18.
As commercially supplied, the stem members 14 and stem collars 18 are supplied as a unit, connected together prior to being supplied to the surgeon.
Reference is made to the implant systems disclosed in U.S. Pat. Nos. 5,683,472 and 6,126,693, which are both incorporated by reference herein in their entireties.
Although the illustrated prior art system is versatile and economic, in some instances, it may be desirable to implant the femoral components with a neutral valgus angle.